Method for recuperating vent gas coming from an ozonization reactor

ABSTRACT

Process for recovering and reusing effluent gas from an ozonization reactor in a plant for treating effluent, especially sewage, which includes a biological reactor, a clarifier and an ozonization reactor. The effluent gas from the ozonization reactor is brought, as a cocurrent, into contact with a stream of water containing compounds that can be oxidized by the ozone, so that the residual ozone is removed and efficiently consumed. The effluent gas thus stripped of the residual ozone is then, optionally, pressurized using a fluid taken from the plant so as to produce a gas/liquid mixture. The gas liquid mixture is converted into an emulsion of fine bubbles, and the emulsion is introduced into the biological reactor, preferably near the bottom of the latter.

FIELD OF INVENTION

The present invention relates to a process and to an apparatus which aredesigned for recovering and reusing the effluent gas from an ozonizationreactor, more particularly in a plant for treating effluent, especiallysewage.

BACKGROUND OF THE INVENTION

It is well known in the treatment of effluent, and more particularly ofsewage, to carry out an oxidization reaction using ozone, the ozonebeing dissolved in the effluent to be oxidized or in some of the sludgefrom the biological reactor or coming from the clarifier, using anozonization reactor often called an ozonization contactor. Sewagetreatment plants are also known which comprise at least one biologicalreactor, at least one clarifier and at least one ozonization reactor, inthis order or in another. The present invention applies moreparticularly to this type of plant. It should be recalled that if theozonization reactor treats water coming from the clarifier, it issometimes necessary to include an additional treatment step offiltration (over sand) between the said clarifier and the saidozonization reactor, this being done so as to remove the suspendedmatter from the effluent and thus improve the effectiveness of theozonization treatment.

After being brought into contact in the ozonization reactor with theeffluent to be treated or with some of the sludge from the biologicalreactor or coming from the clarifier, the effluent gas which escapesfrom the reactor contains a certain amount of residual ozone. It isnecessary to avoid discharging an effluent gas having too high an ozonecontent into the atmosphere.

To avoid such a discharge, it has been envisaged to recover the residualozone for the purpose of recycling it at the head of the contactor.Thus, WO-95/05340 describes a water purification process using ozone, inwhich the undissolved residual ozone is collected in order to be reusedin the water treatment. Likewise, EP-A-0,047,925 describes a process andan apparatus for the treatment of water using ozone, in which theeffluent gas, which contains undissolved oxygen and residual nitrogen,is recovered and recycled into the ozonization reactor. Furthermore,Patent FR-A-2,490,208 describes a process in which the ozonized gascoming from an ozonization reactor is injected directly into thebiological treatment tank, without a prior step of stripping theresidual ozone and without the implementing apparatuses being described.Finally, Patent Application FR 97/09882 describes a process and anapparatus for the purification of sewage, comprising an additionaltreatment of the sludge by ozonization. The gaseous effluent from theozonization reactors is collected so as to reuse it for treating thesewage or any other liquid resulting from the treatment of this sewage.Claims 12 to 14 of this prior application rely on prior destruction ofthe ozone before its reuse or reutilization. However, such recyclingrequires a considerable consumption of energy, given that the effluentgas containing the residual ozone has to be pressurized before it isrecycled.

Under these conditions, an operation of systematically destroying theresidual ozone is more generally carried out. In this regard, referencemay be made to the publication "Memento Technique de l'Eau [WaterTechnical Memorandum]", 50^(th) anniversary issue 1989, second edition,published by Degremont, pages 907-909. Of course, this destruction hasthe drawback of removing, without being used, a not insignificant amountof ozone, which increases the costs.

BRIEF SUMMARY OF THE INVENTION

The object of the present invention is to provide a process and anapparatus for recovering and reusing the effluent gas which do not havethe drawbacks of the prior solutions mentioned above. The objective ofthe invention is, on the one hand, to recover the residual ozone inorder to use it effectively in the ozonization reactor and, on the otherhand, to use the oxygen contained in the effluent gas, thus stripped ofthe residual ozone, for the biological treatment of the effluent.

Consequently, the subject of the present invention is firstly a processfor recovering and reusing effluent gas from an ozonization reactor in aplant for treating effluent, especially sewage, which comprises, at thevery least, in this order or in another, a biological reactor, aclarifier or any other water/sludge separation device and an ozonizationreactor, characterized in that:

the effluent gas from the ozonization reactor is brought into contactwith a stream of water containing components that can be oxidized by theozone, so that most of the residual ozone is removed and used;

the effluent gas stripped of most of the residual ozone is then,optionally, pressurized using a fluid taken from the plant; this fluidpossibly being sludge coming from the clarifier or from the biologicalreactor, water from the clarifier or water coming from the sewagetreatment plant, so as to produce a gas/liquid mixture;

the said gas/liquid mixture is converted into an emulsion of finebubbles; and

the said emulsion is introduced into the biological reactor, preferablynear the bottom of the latter.

According to the present invention, the stream of water brought intocontact with the effluent gas from the ozonization reactor may be afluid from the plant, or any other liquid from the plant which consumesozone effectively, such as the sludge from the biological reactor orcoming from the clarifier. This fluid may be raw water or water takenfrom the clarifier, or any other liquid produced by the plant. If thewater feeding the contactor bringing the effluent gas into contact withthe said stream of water is the same as that treated in the ozonizationreactor, a portion or all of the said stream of water containing thematter that could be oxidized by the ozone may be brought into contactwith the effluent gas in the contactor, the other portion being feddirectly into the reactor 14.

Secondly, the present invention relates to means for recovering andreusing the effluent gas from an ozonization reactor in a plant fortreating effluent, especially sewage, for the implementation of theprocess as specified above, which comprises, at the very least, in thisorder or in another, a biological reactor, a clarifier (or any otherwater/sludge separation device), an ozonization reactor, this plantbeing characterized in that it comprises:

a gas and liquid contactor in which the effluent gas from the reactor isbrought into contact with a stream of water which may be raw waterbefore biological treatment, or clarified water after biologicaltreatment; an alternative is the use of sludge from the clarifier orcoming from the biological reactor or even any other liquid produced bythe plant;

optionally, a water ejector for pressurizing the effluent gas from thesaid contactor and for producing a gas/liquid mixture, the water used inthe said ejector for pressurizing the effluent gas preferably being afluid taken from the plant; this fluid possibly being sludge coming fromthe clarifier or from the biological reactor; water from the clarifieror water coming from the sewage treatment plant;

means for converting the said gas/liquid mixture into an emulsion offine bubbles; and

means for introducing the said gas/liquid mixture of fine bubbles,preferably near the bottom of the biological reactor.

According to one characteristic of this plant, the means for introducingthe emulsion of fine bubbles into the biological reactor consist of arecirculating pump which takes a stream of liquid from the said reactorand of a vertical channel or pipe, in which the outlet of the saidstatic mixer emerges, the lower end of which vertical channel or pipeemerges at the bottom of the said biological reactor, the said untreatedstream taken from this reactor flowing into the said vertical channel orpipe so as to drive the emulsion of fine bubbles down to the bottom ofthe biological reactor.

According to another characteristic of this invention, the water ejectoris placed directly in the biological reactor and the emulsion of finebubbles is created from sludge directly taken from the biologicalreactor by means of a pump, the same pump and the same driving fluidserving both to form the emulsion and to bring it into contact with thesludge present in the biological tank.

Further characteristics and advantages of the present invention willemerge from the description given below with reference to the appendeddrawing, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of a plant according to thepresent invention;

FIG. 2 is a diagrammatic representation, similar to FIG. 1, of analternative form of this plant;

FIG. 3 is a diagrammatic representation, similar to FIG. 1, of anotheralternative form of this plant; and

FIG. 4 is a diagrammatic representation, similar to FIG. 1, of anotheralternative form of this plant.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIGS. 1 to 3 show a plant used for ozonizing the water coming from theclarifier.

FIG. 4 shows a plant used for ozonizing the sludge coming from theclarifier.

Referring to the drawings, and more particularly to FIG. 1, it may beseen that the plant to which the present invention applies is a plantfor treating effluent, especially sewage, which comprises, in series, abiological reactor 10, a clarifier 12 and an ozonization reactor 14. Allthese devices are well known and thus it seems pointless to describethem.

According to the present invention, the effluent gas from theozonization reactor 14, which contains undissolved oxygen and residualozone, is (in this non-limiting embodiment) brought into contact, as acountercurrent, with water from the biological reactor 10 and from theclarifier 12. For this purpose, a contactor 16 is used which, as may beseen in the figure, preferably consists of a packed column of known typethrough which some of the clarified water, injected into it by means ofa pump 13, trickles as a countercurrent to the effluent gas coming fromthe ozonization reactor 14. It should be noted that a portion or all ofthe water to be treated in the ozonization reactor 14 can pass throughthe contactor 16. The major portion of the residual ozone is thusconsumed by the most rapid ozonization reactions. This technique has thedouble advantage of eliminating most of the residual ozone present inthe effluent gas coming from the ozonization reactor 14, which would beharmful for the biological treatment if it were to be reintroduced inlarge amounts, and of improving the overall efficiency of the use of theozone for oxidation.

In the alternative form of the invention illustrated in FIG. 2, theeffluent gas from the ozonization reactor 14 is brought into contact, asa cocurrent, with a stream of raw water in the contactor 16.

An effluent gas is therefore obtained which is stripped of most of theozone and essentially contains only oxygen that has not dissolved in thereactor 14. According to the present invention, this oxygen is recoveredin order to be recycled into the biological reactor 10. For thispurpose, it has to be pressurized, unless the ozonization reactor 14 isunder pressure. According to the invention, a water ejector is used forthis purpose which produces a gas/liquid mixture under pressure. Showndiagrammatically at 18 in the figure is such a water ejector, which isof a known type and which consequently will not be described in detail.Preferably, the driving water used in the ejector 18 is clarified watertaken from the clarifier 12 using a pump 15.

The gas/liquid mixture, thus pressurized, from the water ejector 18 (ordirectly from the ozonization reactor 14, when the latter is underpressure) is then converted into an emulsion of fine bubbles. A staticmixer 20 (FIG. 1) can be used for this purpose. The emulsion thusobtained is then introduced into the biological reactor 10. Theapparatus according to the invention uses here, for this purpose, meanswhich consist of a recirculating pump 17, which takes a stream of waterfrom the biological reactor 10, and of a vertical channel or pipe 22 inwhich the outlet pipe 19 of the static mixer 20 emerges, the lower end24 of which vertical channel or pipe emerges at the bottom of thebiological reactor. By virtue of these means, the emulsion of finebubbles produced is driven down to the bottom of the biological reactor10 and distributed within the mass of effluent present in this reactor.

In the alternative form of the invention illustrated in FIG. 3, theozone-containing gas is brought into contact with the raw water in acocurrent contactor 16. Moreover, this alternative form of the inventionis identical to the embodiment described with reference to FIG. 1. Theemulsion may be obtained from the driving water coming from thebiological reactor 10. The emulsion thus obtained is then introducedinto the biological reactor. For this purpose, the water ejector 18 maybe placed directly in the biological reactor. According to analternative form, the feed pump 15 delivering the driving water to theejector may possibly be placed in the biological reactor.

In the alternative form of the invention illustrated in FIG. 4, theeffluent gas, containing undissolved oxygen and residual ozone, isbrought into contact with the sludge 25 coming from the, clarifier 12.As may be seen in the figure, a contactor 16 is used, in which thesludge is introduced by means of a pump 13. It should be noted that aportion or all of the sludge to be treated in the ozonization reactor 14may pass through the contactor 16. The other devices for reusing the gasare not described since they are identical to those in FIG. 3.

It will have been understood from reading the description above that theinvention makes it possible to remove from the effluent gas most of theresidual ozone, while avoiding having to discharge this ozone into theatmosphere or to remove it, which ozone is used efficiently for theoxidation reactions, in order to recycle the undissolved oxygencontained in the effluent gas and to carry out aerobic treatments in thebiological reactor. It will have been noted that the apparatus formingthe subject of the invention does not require a compressor for recyclingthe effluent gas, the latter being pressurized, if necessary, by a waterejector, which is inexpensive and simple to manufacture, and onlyrequiring a low consumption of energy because of operation at lowpressure. Moreover, the production and use of fine bubbles make itpossible to reduce the hydrostatic loads and to obtain a very highsurface area for gas/liquid transfer throughout the mass of effluentcontained in the biological reactor, while at the same time producingvigorous stirring within this mass, favouring the distribution of theoxygen bubbles.

Moreover, depending on the methods of implementation of the saidinvention, various advantages may be expected. This is because thesludge treatment in the ozonization reactor can result in a reduction inthe mass of sludge produced by the water treatment process, and thetreatment of the water coming from the clarifier by the ozone and therecycling of this water into the biological reactor allows a reductionin the physico-chemical parameters of the treated water coming from thewater treatment plant.

Of course, it remains the case that this invention is not limited to theembodiments or methods of implementation described and shown here,rather it encompasses all alternative forms thereof provided that theyfall within the scope of the appended claims.

What is claimed is:
 1. Process for recovering and reusing effluent gasfrom an ozonization reactor in an effluent treating plant which includesa biological reactor, a water/sludge clarifier and an ozonizationreactor, the steps comprising:bringing the effluent gas from theozonization reactor into contact with a stream of water containingcompounds that can be oxidized by residual ozone in the effluent gas sothat the residual ozone is removed and efficiently consumed; mixing theeffluent gas thus stripped of the residual ozone with a fluid taken fromthe plant so as to produce a gas/liquid mixture; converting saidgas/liquid mixture into an emulsion of fine bubbles; and introducingsaid emulsion into a bottom portion of the biological reactor. 2.Process according to claim 1, wherein said fluid taken from the plantand mixed with the stripped effluent gas is pressurized.
 3. Processaccording to claim 1, wherein the stream of water brought into contactwith the effluent gas from the ozonization reactor is effluent. 4.Process according to claim 1, wherein the stream of water brought intocontact with the effluent gas from the ozonization reactor is sludgefrom the biological reactor.
 5. Process according to claim 1, whereinthe stream of water brought into contact with the effluent gas from theozonization reactor is sludge from the clarifier.
 6. Process accordingto claim 1, wherein the fluid brought into contact with the effluent gasfrom the ozonization reactor is treated water or water undergoingtreatment.
 7. Process according to claim 1, wherein the fluid broughtinto contact with the effluent gas from the ozonization reactor is atleast part of the feed stream for the ozonization reactor.
 8. Processaccording to claim 1, wherein the stream of water brought into contactwith the effluent gas from the ozonization reactor is water taken fromsaid clarifier.
 9. Process according to claim 1, wherein the fluid whichis mixed with the stripped effluent gas is water taken from saidclarifier and filtered beforehand.
 10. Process according to claim 1,wherein the fluid which is mixed with the stripped effluent gas issludge from the biological reactor.
 11. Process according to claim 1,wherein the fluid which is mixed with the stripped effluent gas issludge from the clarifier.
 12. Process according to claim 1, wherein thefluid which is mixed with the stripped effluent gas is treated waterleaving the effluent ozonization reactor.
 13. Apparatus for recoveringand reusing the effluent gas from an ozonization reactor in a treatmentplant for treating effluent which comprises a biological reactor, aclarifier, and an ozonization reactor, this apparatus furthercomprising:a contactor in which the effluent gas from the ozonizationreactor is brought into contact with a stream of liquid, said liquidbeing effluent or a liquid produced by the treatment plant; an ejectorfor pressurizing the effluent gas from said contactor so as to produce agas/liquid mixture; means for converting said gas/liquid mixture into anemulsion of fine bubbles; and means for introducing said gas/liquidmixture near the bottom of the biological reactor.
 14. Apparatusaccording to claim 13, wherein said contactor is a packed column throughwhich water trickles as a countercurrent to the effluent gas. 15.Apparatus according to claim 13 wherein said means for converting is astatic mixer which is placed downstream of the ejector.
 16. Apparatusaccording to claim 13 wherein a liquid inlet to the ejector is connectedto and in fluid communication with an outlet of said clarifier. 17.Apparatus according to claim 13, wherein a liquid inlet to the ejectoris connected to and in fluid communication with an outlet of saidbiological reactor.
 18. Apparatus according to claim 13, wherein aliquid inlet to the ejector is connected to and in fluid communicationwith an outlet of said ozonization reactor.
 19. Apparatus according toclaim 13, wherein a liquid inlet to the ejector is connected to and influid communication with an outlet of said clarifier.
 20. Apparatusaccording to claim 13, wherein the means for introducing the emulsion offine bubbles into the biological reactor consist of a recirculating pumpwhich takes water from said biological reactor and of a vertical pipeinto which the outlet of said means for converting emerges, the lowerend of the pipe emerging near the bottom of said biological reactor, astream of water taken from the biological reactor flowing into said pipeso as to drive the emulsion of fine bubbles down to the bottom of thebiological reactor.
 21. Apparatus according to claim 13, wherein themeans for introducing the emulsion of fine bubbles into the biologicalreactor consist of a recirculating pump which is placed inside thebiological reactor and which takes water from the biological reactor,and from the ejector placed in the biological reactor.